Optical fiber and optical communication system using this optical fiber

ABSTRACT

The present invention resides in an optical fiber able to form an optical transmitting line for wavelength division multiplexing transmission in a wavelength band of 1.5 μm using a Raman amplifier, and an optical communication system using this optical fiber. The optical fiber has an effective core area from 40 μm 2  to 60 μm 2  in a set wavelength band of at least one portion of a wavelength band of 1.5 μm; a dispersion value from 4 to 10 ps/nm/km at a wavelength of 1.55 μm; a dispersion slope set to a positive value equal to or smaller than 0.04 ps/nm 2 /km in a wavelength band of 1.55 μm; and a zero dispersion wavelength equal to or smaller than 1.4 μm. Further, a cutoff wavelength is set to be equal to or smaller than 1.5 μm at a length of 2 m, and a bending loss is set to be equal to or smaller than 5 dB/m at a diameter of 20 mm in the wavelength band of 1.5 μm. In a refractive index profile of the optical fiber, for example, a relative refractive index difference Δ1 of a first glass layer as an innermost layer with respect to a reference layer, and a relative refractive index difference Δ3 of the refractive index of a third glass layer as a third layer from an inner side with respect to the reference layer are set to be positive. Further, a relative refractive index difference Δ2 of a second glass layer as a second layer from the inner side with respect to the reference layer is set to be negative.

FIELD OF THE INVENTION

This invention relates to an optical fiber used in optical transmissionsuch as wavelength division multiplexing (WDM) transmission, etc. in awavelength band of e.g., 1.5 μm, etc., and optical communication systemsusing this optical fiber.

BACKGROUND OF THE INVENTION

A communication information capacity tends to be greatly increased asinformation society is developed. Techniques of the wavelength divisionmultiplexing transmission (WDM transmission) and time divisionmultiplexing (TDM) transmission are noticed as such information isincreased. This wavelength division multiplexing transmission uses asystem for transmitting signals of plural wavelengths by one opticalfiber. Therefore, this system is an optical transmitting system suitablefor high capacity and high bit-rate transmission. The wavelengthdivision multiplexing transmission technique is vigorously studied atpresent.

It is considered at present that the wavelength division multiplexingtransmission is performed in a wavelength band of 1.55 μm as a gain bandof an erbium-doped optical fiber amplifier. The wavelength band of 1.55μm is a wavelength band with 1550 nm in wavelength approximately as acenter, e.g., as in a wavelength band from 1530 nm to 1570 nm.

However, there are problems of an increase in power of an optical signaland a non-linear phenomenon due to an interaction between signals, etc.to perform the wavelength division multiplexing transmission. Therefore,for example, it is reported in a society report document OFC' 97 TuNlbof Japan, etc. to consider that a non-linear refractive index difference(n₂) is reduced and restrained to restrain the non-linear phenomenon.

It is also noticed to consider that an effective core area (A_(eff)) ofthe optical fiber is increased together with this reduction in thenon-linear refractive index difference. Distortion φ_(NL) of a signaldue to the non-linear phenomenon is generally represented by thefollowing formula (1). Therefore, when the effective core area of theoptical fiber is increased, the waveform distortion of a signal due tothe non-linear phenomenon can be reduced.

φ_(NL)=(2π×n ₂ ×L _(eff) ×P)/(λ×A _(eff))  (1)

In the formula (1), π, n₂, L_(eff), P and λ respectively designate aratio of the circumference of a circle to its diameter, a non-linearrefractive index, an effective optical fiber length, signal power and asignal optical wavelength.

Accordingly, it is very important to enlarge the effective core area inthe optical fiber used for e.g. the wavelength multiplexingtransmission, and this enlargement is very noticed as reported insociety report documents OFC'96 WK15 and OFC'97 YuN2 of Japan.

SUMMARY OF THE INVENTION

The present invention provides an optical fiber and an opticalcommunication system using this optical fiber.

The optical fiber of the invention comprises:

an effective core area from 40 μm² to 60 μm² in a set wavelength band ofat least one portion of a wavelength band of 1.5 μm;

a dispersion value set to 4 ps/nm/km or more and 10 ps/nm/km or less ata wavelength of 1.55 μm;

a dispersion slope set to a positive value equal to or smaller than 0.04ps/nm²/km in a wavelength band of 1.55 μm; and

a zero dispersion wavelength equal to or smaller than 1.4 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described inconjunction with drawings, in which:

FIG. 1A is an explanatory view showing the construction of a refractiveindex profile in a first embodiment of an optical fiber in theinvention.

FIG. 1B is an explanatory view showing a sectional construction of theoptical fiber in the first embodiment of the optical fiber in theinvention.

FIG. 2 is an explanatory view showing a refractive index profileconstruction in a second embodiment of the optical fiber in theinvention.

FIG. 3 is an explanatory view showing a refractive index profileconstruction in a third embodiment of the optical fiber in theinvention.

DETAILED DESCRIPTION

In an optical fiber, a diffusion slope is generally increased when aneffective core area is enlarged. The problem of a difference inchromatic dispersion every wavelength is caused by the increase indispersion slope, and becomes a great obstacle in wavelength divisionmultiplexing transmission. Therefore, a reduction in dispersion slope isa very important.

It is studied in recent years that a Raman amplifier is applied insteadof the wavelength division multiplexing transmission using theerbium-doped optical fiber amplifier, and the wavelength divisionmultiplexing transmission is performed in e.g., a wavelength band of 1.5μm. The wavelength band of 1.5 μm is a wavelength band with 1500 nm inwavelength approximately as a center, e.g. as in a wavelength band from1500 nm to 1650 nm. Hereafter, the term of the wavelength band of 1.5 μmis used as this meaning.

The Raman amplifier is an optical amplifier utilizing Ramanamplification described below. The Raman amplification is an amplifyingmethod of an optical signal utilizing a so-called Raman amplifyingphenomenon. In the Raman amplifying phenomenon, when pumping light asstrong light is incident to the optical fiber, a gain appears about 100nm on a long wavelength side from a pumping light wavelength by inducedRaman scattering, and signal light in a wavelength area having this gainis amplified when this signal light is incident to the optical fiber inthis pumped state.

Therefore, when the wavelength division multiplexing transmission in awavelength band of 1.5 μm is performed by using the Raman amplifier,pumping light having about 1.4 μm in wavelength is incident to theoptical fiber.

However, in the optical fiber conventionally considered for thewavelength division multiplexing transmission, wavelength dispersion ata wavelength of 1.55 μm approximately ranges from −4 ps/nm/km to +6ps/nm/km, and its dispersion slope is 0.05 ps/nm²/km or more. Therefore,in the optical fiber conventionally considered for the wavelengthdivision multiplexing transmission, a zero dispersion wavelength becomes1.4 μm or more so that an interference of the pumping light of about 1.4μm in wavelength and four-wave mixing, etc. is caused.

In an optical fiber and an optical communication system in one aspect ofthe invention, no problem of an interference with pumping light, etc. isalmost caused even when the wavelength division multiplexingtransmission in a wavelength band of 1.5 μm is performed by using e.g.,the Raman amplifier, and the optical fiber and the optical communicationsystem have a low dispersion slope with low non-linearity.

Concrete embodiments of the invention will next be explained on thebasis of the drawings. FIG. 1A shows a refractive index distributionprofile in a first embodiment of an optical fiber in the invention. Theprofile of the refractive index distribution of the optical fiber can beset to refractive index profiles in various modes. However, in the firstembodiment, a refractive index profile as shown in FIG. 1A is adopted.This refractive index profile is relatively simple in structure, and iseasily designed and controlled in refractive index structure.

The optical fiber of the first embodiment has multiple (four layershere) glass layers (a first glass layer 1, a second glass layer 2, athird glass layer 3 and a reference layer 6) adjacent to each other andhaving different compositions. As shown in FIG. 1B, these glass layersare formed in a concentric shape. The reference layer 6 as an outermostlayer is a layer constituting a reference of the refractive indexdistribution among the four glass layers. Three glass layers constructedby the first glass layer 1, the second glass layer 2 and the third glasslayer 3 are formed inside this reference layer 6.

In the optical fiber of the first embodiment, a maximum refractive indexof the first glass layer 1 formed on an innermost side of the opticalfiber, and a maximum refractive index of the third glass layer 3 as athird layer from an inner side are set to be higher than the refractiveindex of the reference layer 6. Further, in the optical fiber of thefirst embodiment, a minimum refractive index of the second glass layer 2as a second layer from the inner side of the optical fiber is set to belower than the refractive index of the reference layer 6. A refractiveindex distribution shape of the first glass layer 1 is formed in αshape.

In the optical fiber of the first embodiment, Δ1>Δ3>Δ2 is formed when amaximum relative refractive index difference of the first glass layer 1with respect to the reference layer 6 is set to Δ1, a minimum relativerefractive index difference of the second glass layer 2 with respect tothe reference layer 6 is set to Δ2, and a maximum relative refractiveindex difference of the third glass layer 3 with respect to thereference layer 6 is set to Δ3.

In this specification, the refractive index of a maximum refractiveindex portion of the first glass layer is set to n1, the refractiveindex of a minimum refractive index portion of the second glass layer isset to n2, the refractive index of a maximum refractive index portion ofthe third glass layer is set to n3, and the refractive index of thereference layer is set to n6. The respective relative refractive indexdifferences Δ1, Δ2 and Δ3 are respectively defined by the followingapproximate formulas (2) to (4).

Δ1≅{(n1−n6)/n6}×100  (2)

Δ2≅{(n2−n6)/n6}×100  (3)

Δ3≅{(n3−n6)/n6}×100  (4)

The optical fiber of the first embodiment has the refractive indexprofile shown in FIG. 1A, and also has the following construction.Namely, the optical fiber of the first embodiment has a construction inwhich an effective core area ranges from 40 μm² to 60 μm² in a setwavelength band of at least one portion of a wavelength band of 1.5 μm.This optical fiber also has a construction in which a dispersion valueat a wavelength of 1.55 μm is set to 4 ps/nm/km or more and is set to 10ps/nm/km or less. This optical fiber also has a construction in which adispersion slope in the wavelength band of 1.55 μm is set to a positivevalue equal to or smaller than 0.04 ps/nm²/km. This optical fiberfurther has a construction in which a zero dispersion wavelength is setto 1.4 μm or less. For example, the set wavelength band is a wavelengthband of 1.55 μm.

Further, the optical fiber of the first embodiment has a construction inwhich a cutoff wavelength at a length of 2 m is set to 1.5 μm or less,and a bending loss at a diameter of 20 mm in the wavelength band of 1.5μm is set to 5 dB/m or less.

The present inventors have considered that the optical fiber of thefirst embodiment is applied to wavelength division multiplexingtransmission in the wavelength band of 1.5 μm, and the followingconsideration is taken into account with respect to the refractive indexprofile shown in FIG. 1A.

Namely, respective relative refractive indexes Δ1, Δ2, Δ3, α andrespective diameters a, b, c are set to parameters, and these values areset to various values. When a single mode condition is satisfied, aprofile range is searched such that the dispersion slope (an averagevalue of the dispersion slope) in the wavelength band of 1.55 μm amongthe wavelength band of 1.5 μm becomes a positive value equal to orsmaller than 0.03 ps/nm²/km. An optimum profile of the first embodimentis calculated from the relation of the effective core area and a bendingloss value in this profile range.

As a result, when no relative refractive index difference Δ1 is set tolie within a range equal to or smaller than 0.6%, it has been found thatit is difficult to set the effective core area to 40 μm² or more whenthe dispersion slope is set to a positive value equal to or smaller than0.03 ps/nm²/km. Further, it has been found that the bending loss becomesa value greater than 5 dB/m when the relative refractive indexdifference Δ1 is set to be smaller than 0.5%. Therefore, the range ofthe relative refractive index difference Δ1 is set to a range from 0.5%to 0.6%.

The relative refractive index difference Δ1 is set to lie within theabove range and the constant α not increasing the dispersion slope iscalculated when the effective core area is enlarged. It is then judgedthat the constant α is suitably set to 5.0 or more. In this condition,the refractive index profile is calculated such that the effective corearea can be set to 40 μm² or more and 60 μm or less, and the dispersionslope can be set to a positive value equal to or smaller than 0.04ps/nm²/km while the bending loss value at the diameter of 20 mm is heldto be equal to or smaller than 5 dB/m.

As a result, when the relative refractive index difference Δ2 is set tobe smaller than −0.4%, it is difficult to set the effective core area tobe equal to or greater than 40 μm² , and the bending loss value at thediameter of 20 mm also becomes a value greater than 5 dB/m. Further,when the relative refractive index difference Δ2 is set to be greaterthan −0.1%, the dispersion slope becomes a value greater than 0.04ps/nm²/km. Therefore, the range of the relative refractive indexdifference Δ2 is set to a range from −0.4% to −0.1%.

When the relative refractive index difference Δ3 is set to be smallerthan 0.1%, it is difficult to set the effective core area to be equal toor greater than 40 μm², and the bending loss value at the diameter of 20mm also becomes a value greater than 5 dB/m. Further, when the relativerefractive index difference Δ3 is set to be greater than 0.4%, a cutoffwavelength λc becomes larger than 1.5 μm. Therefore, the range of therelative refractive index difference Δ3 is set to a range from 0.1% to0.4%.

The refractive index profile in each of concrete examples 1 to 4 shownin table 1 is determined from the above consideration results.

TABLE 1 BENDING LOSS CORE (MEASURING Δ 1 Δ 2 Δ 3 DIAMETER DISPERSIONSLOPE Aeff λ c VALUE AT 20 mm φ) λ o UNIT % α % % a:b:c μm ps/nm/kmps/nm²/km μm² nm dB/m nm CONCRETE 0.58 10 −0.30 0.20 1:1.9:2.5 12.9 7.40.024 41.1 1446 3.5 1242 EXAMPLE 1 CONCRETE 0.57 10 −0.30 0.25 1:1.8:2.412.8 6.3 0.026 43.5 1469 3.0 1038 EXAMPLE 2 CONCRETE 0.55 12 −0.20 0.251:1.8:2.3 12.4 7.1 0.034 48.6 1341 2.0 1341 EXAMPLE 3 CONCRETE 0.52  6−0.20 0.20 1:1.7:2.2 13.2 6.9 0.039 52.4 1446 4.0 1373 EXAMPLE 4 λ =1550 nm

The table 1 shows setting examples of the respective relative refractiveindex differences Δ1, Δ2, Δ3, and examples of the value of the constantα, and a ratio of a:b:c, a core diameter and simulation results ofcharacteristics of the optical fiber when an outside diameter of thefirst glass layer 1 is set to a, an outside diameter of the second glasslayer 2 is set to b, and an outside diameter of the third glass layer 3is set to c.

In the table 1 and tables shown below, respective values of the corediameter and the optical fiber characteristics show the followingvalues. Namely, the core diameter shows the outside diameter of thesecond layer (the value of b in a corresponding figure among FIGS. 1 to3). Dispersion shows a dispersion value at a wavelength of 1.55 μm.Slope shows an average value of the dispersion slope (dispersiongradient) in a wavelength band of 1.55 μm, and becomes a value equal tothe dispersion slope in a wavelength band of 1.5 μm. A_(eff) shows aneffective core area when 1.55 μm signal is propagated. λc shows a cutoffwavelength at a length of 2 m. Bending loss shows a value of the bendingloss at a diameter of 20 mm with respect to light of 1.55 μm inwavelength. λ₀ shows a zero dispersion wavelength.

In the optical fiber of the first embodiment, the zero dispersionwavelength can be set to be equal to or smaller than 1.4 μm by therefractive index profile shown in FIG. 1A and the table 1, and thisoptical fiber has characteristics shown in the table 1 at a wavelengthof 1.55 μm and in a wavelength band including this wavelength 1.55 μm.Namely, in the optical fiber of the first embodiment, the dispersionvalue at the wavelength of 1.55 μm is set to 4 ps/nm/km or more, and 10ps/nm/km or less, and the dispersion slope in the wavelength band of1.55 μm is set to a positive value equal to or smaller than 0.04ps/nm²/km so that the zero dispersion wavelength can be set to be equalto or smaller than 1.4 μm.

Accordingly, in the optical fiber of the first embodiment, when Ramanamplification is performed in the wavelength band of 1.5 μm, it ispossible to restrain the generation of an interference of pumping lightof about 1.4 μm in wavelength and four-wave mixing, etc.

Further, since the dispersion value at the wavelength of 1.55 μm is setto be equal to or smaller than 10 ps/nm/km as mentioned above, nooptical fiber of the first embodiment has large local dispersion as in acase in which the dispersion value is set to be greater than 10ps/nm/km. Accordingly, the optical fiber of the first embodiment canrestrain distortion due to dispersion, and can also reduce thedifference in dispersion between wavelengths.

Further, the optical fiber of the first embodiment can reduce thedifference in dispersion between wavelengths since an absolute value ofthe dispersion slope is reduced by setting the dispersion slope in thewavelength band of 1.55 μm to a positive value equal to or smaller than0.04 ps/nm²/km. Accordingly, the optical fiber of the first embodimentbecomes an optical fiber suitable for the wavelength divisionmultiplexing transmission in the wavelength band of 1.5 μm to which theRaman amplifier is applied.

Further, since the absolute value of the dispersion slope in the opticalfiber of the first embodiment is small, the dispersion slope of theoptical fiber of the first embodiment can be easily compensated byconnecting e.g., a dispersion slope compensating fiber (DSCF), etc.conventionally developed to the optical fiber of the first embodiment.

As is well known, there are a Raman amplifier of a distribution type anda Raman amplifier of a concentration type in the Raman amplifier. Whenthe Raman amplifier of the concentration constant type is applied to thewavelength division multiplexing transmission, no nonlinear phenomenonwithin the optical fiber can be neglected. In this case, in the opticalfiber of the first embodiment, the effective core area is set to 40 μm²or more equal to or greater than that of the conventional optical fiberfor the wavelength division multiplexing transmission in the setwavelength band of at least one portion of the wavelength band of 1.5μm. Accordingly, the optical fiber of the first embodiment can alsorestrain signal light distortion due to the nonlinear phenomenon byperforming the wavelength division multiplexing transmission in this setwavelength band.

Further, when the Raman amplifier of the distribution constant type isapplied, maximum input power of the optical fiber can be reduced andrestrained so that the signal light distortion due to the nonlinearphenomenon within the optical fiber can be reliably restrained.

When the effective core area is too large, a reduction in efficiency ofthe Raman amplifier is caused. However, in the optical fiber of thefirst embodiment, the effective core area is set to 60 μm² or less inthe set wavelength band of at least one portion of the wavelength bandof 1.5 μm. Accordingly, in the optical fiber of the first embodiment,the reduction in efficiency of the Raman amplifier can be restrained byperforming the wavelength division multiplexing transmission using theRaman amplifier in this set wavelength band.

Since the cutoff wavelength is set to 1.5 μm in wavelength or less inthe optical fiber of the first embodiment, a single mode operation canbe precisely performed in a wavelength band equal to or greater than 1.5μm in wavelength. Further, the optical fiber of the first embodiment canalso restrain the bending loss when the optical fiber is formed as acable.

Accordingly, the optical fiber of the first embodiment becomes anoptical fiber suitable for the wavelength division multiplexingtransmission in the wavelength band of 1.5 μm and able to efficientlyperform the Raman amplification. An optical communication systemapplying the optical fiber of the first embodiment thereto as an opticaltransmission line can be set to a wavelength division multiplexingtransmission system in the wavelength band of 1.5 μm, etc. using e.g.the Raman amplification with high quality.

When the restriction of an influence of the four-wave mixing isseriously considered in the first embodiment and second and thirdembodiments shown below, it is desirable to set the dispersion value to6 ps/nm/km or more as shown in each table.

FIG. 2 shows a refractive index profile in a second embodiment of theoptical fiber in the invention. The second embodiment approximately hasa construction similar to that of the first embodiment. The secondembodiment characteristically differs from the first embodiment in thata glass layer having a refractive index lower than that of a referencelayer 6 is arranged between a third glass layer 3 and the referencelayer 6. This glass layer having the low refractive index is a fourthglass layer 4. The fourth glass layer 4 is adjacently arranged on anouter circumferential side of the third glass layer 3.

In this specification, a relative refractive index difference Δ4 of thefourth glass layer 4 with respect to the reference layer 6 is defined byan approximate formula (5) shown below when the refractive index of aminimum refractive index portion of the fourth glass layer 4 is set ton4. In the second embodiment, the relative refractive index differenceΔ4 is set to approximately range from −0.2% to −0.1%.

Δ4≅{(n4−n6)/n6}×100  (5)

Table 2 shows the relative refractive index difference Δ4 of the opticalfiber in each of concrete examples 5 to 8 of the second embodiment, anoutside diameter ratio a:b:c:d of the first to fourth glass layers, andcharacteristics of the optical fiber. In the concrete examples 5 to 8,relative refractive index differences Δ1, Δ2, Δ3 and constant α are setto values similar to those in the optical fiber of the concrete example2 shown in the table 1. The outside diameter ratio a:b:c:d of the firstto fourth glass layers is a ratio when the outside diameter of the firstglass layer 1 is a, the outside diameter of the second glass layer 2 isb, the outside diameter of the third glass layer 3 is c, and the outsidediameter of the fourth glass layer 4 is d.

TABLE 2 BENDING LOSS CORE (MEASURING Δ 4 DIAMETER DISPERSION SLOPE Aeffλ c VALUE AT 20 mm φ) λ o UNIT % a:b:c:d μm ps/nm/km ps/nm²/km μm² nmdB/m nm CONCRETE −0.20 1:1.8:2.5:3.0 13.3 7.5 0.031 43.0 1246 3.0 1308EXAMPLE 5 CONCRETE −0.15 1:1.8:2.5:3.0 13.1 7.2 0.028 43.2 1307 3.0 1293EXAMPLE 6 CONCRETE −0.10 1:1.8:2.5:3.0 13.0 6.9 0.027 43.3 1379 3.0 1294EXAMPLE 7 CONCRETE −0.10 1:1.8:2.5:4.0 12.8 7.3 0.030 43.1 1298 3.0 1307EXAMPLE 8 λ = 1550 nm

As shown in the table 2, in the optical fiber of the second embodiment,the cutoff wavelength can be reduced, and it is possible to set anoptical fiber also able to cope with wavelength multiplexingtransmission in a wavelength band of 1.31 μm as well as a wavelengthband of 1.55 μm.

FIG. 3 shows a refractive index profile of a third embodiment of theoptical fiber in the invention. The third embodiment approximately has aconstruction similar to that of the first embodiment. The thirdembodiment characteristically differs from the first embodiment in thata glass layer having a refractive index higher than that of a referencelayer 6 is arranged between a third glass layer 3 and the referencelayer 6. This glass layer having the high refractive index is a fifthglass layer 5.

In the third embodiment, a fourth glass layer 4 is adjacently arrangedon an outer circumferential side of the third glass layer 3, and has arefractive index equal to that of the reference layer 6. The fifth glasslayer 5 is adjacently arranged on an outer circumferential side of thefourth glass layer 4.

In this specification, a relative refractive index difference Δ5 of thefifth glass layer 5 with respect to the reference layer 6 is defined byan approximate formula (6) shown below when the refractive index of amaximum refractive index portion of the fifth glass layer is set to n5.In the third embodiment, the relative refractive index difference Δ5 isset to approximately range from 0.1% to 0.2%.

Δ5≅{(n5−n6)/n6}×100  (6)

Table 3 shows the relative refractive index difference Δ5 of the opticalfiber in each of concrete examples 9 to 12 of the third embodiment, anoutside diameter ratio a:b:c:d:e of the first to fifth glass layers, andcharacteristics of the optical fiber. In the concrete examples 9 to 12,relative refractive index differences Δ1, Δ2, Δ3 and constant α are setto values similar to those in the optical fiber of the concrete example3 shown in the table 1. The outside diameter ratio a:b:c:d:e of thefirst to fifth glass layers is a ratio when the outside diameter of thefirst glass layer 1 is a, the outside diameter of the second glass layer2 is b, the outside diameter of the third glass layer 3 is c, theoutside diameter of the fourth glass layer 4 is d, and the outsidediameter of the fifth glass layer 5 is e.

TABLE 3 BENDING LOSS CORE (MEASURING Δ 5 DIAMETER DISPERSION SLOPE Aeffλ c VALUE AT 20 mm φ) λ o UNIT % a:b:c:d:e μm ps/nm/km ps/nm²/km μm² nmdB/m nm CONCRETE 0.20 1:1.8:2.3:2.8:3.0 12.1 6.5 0.026 49.2 1505 3.01300 EXAMPLE 9 CONCRETE 0.10 1:1.8:2.5:2.8:3.0 12.3 6.7 0.030 48.9 14263.0 1327 EXAMPLE 10 CONCRETE 0.10 1:1.8:2.5:2.8:3.2 12.4 6.4 0.027 49.01495 3.0 1303 EXAMPLE 11 CONCRETE 0.10 1:1.8:2.5:3.0:3:2 12.3 6.9 0.03149.6 1468 3.0 1327 EXAMPLE 12 λ = 1550 nm

As shown in the table 3, the optical fiber of the third embodiment canhave effects similar to those in the first embodiment.

A fabrication example of the optical fiber actually fabricated on thebasis of the above simulation results will next be explained. Thepresent inventors fabricated the actual optical fiber on the basis of adesign of the optical fiber of the concrete example 2 of the table 1.Table 4 shows results of this fabrication.

TABLE 4 BENDING LOSS CORE TRANSMISSION (MEASURING DIAMETER λ ODISPERSION SLOPE λ C Aeff LOSS VALUE AT 20 mm φ) UNIT μm nm ps/nm/kmps/nm²/km nm μm² dB/km dB/m FABRICATION 13.5 1337 6.8 0.027 1414 42.80.24 3.2 EXAMPLE 1 FABRICATION 14.0 1371 7.6 0.029 1476 44.0 0.23 2.4EXAMPLE 2 λ = 1550 nm

As clearly seen from the table 4, similar to design values, the opticalfiber of each fabrication example has low dispersion and a lowdispersion slope, and has low transmission loss. Further, in the opticalfiber of each fabrication example, since the zero dispersion wavelength(λ₀) is equal to or smaller than 1400 nm, no problem of an interferencewith pumping light, etc. is caused even when the wavelength divisionmultiplexing transmission is performed in a wavelength band of 1.5 μm byusing e.g. the Raman amplifier.

The invention is not limited to each of the above embodiments, butvarious kinds of embodiment modes can be adopted. For example, theoptical fiber of the invention may have a refractive index profileexcept for the refractive index profile shown in each of the aboveembodiments. Namely, in the optical fiber of the invention, it issufficient to set the effective core area, the dispersion value and thedispersion slope at least at a set wavelength or in a set wavelengthband in the wavelength band of 1.5 μm to e.g. suitable values as shownin each of the above embodiments, and set the zero dispersion wavelengthto 1.4 μm or less. In this construction, it is possible to construct anoptical fiber and an optical communication system using this opticalfiber in which the wavelength division multiplexing transmission in thewavelength band of 1.5 μm using the Raman amplifier is performed withhigh quality.

In the above examples, the optical fiber and the optical communicationsystem are applied to the wavelength division multiplexing transmissionin the wavelength band of 1.5 μm using the Raman amplifier. However, theoptical fiber and the optical communication system of the invention canbe also applied to the wavelength division multiplexing transmissionusing e.g. an erbium-doped optical fiber amplifier except for the Ramanamplifier. Further, in accordance with the construction of the opticalfiber, the optical fiber and the optical communication system can bealso applied to the wavelength division multiplexing transmission in awavelength band except for the wavelength band of 1.5 μm in addition tothis wavelength band of 1.5 μm.

Further, in the above examples, the wavelength band is set to thewavelength band of 1.55 μm. However, the set wavelength band is notparticularly limited, but is suitably set in conformity with thewavelength band applied to the wavelength division multiplexingtransmission, etc.

What is claimed is:
 1. An optical fiber comprising: an effective corearea from 40 μm² to 60 μm² in a set wavelength band of at least oneportion of a wavelength band of 1.5 μm; a dispersion value set to 4ps/nm/km or more and 10 ps/nm/km or less at a wavelength of 1.55 μm; adispersion slope set to a positive value equal to or smaller than 0.04ps/nm²/km in a wavelength band of 1.55 μm; and a zero dispersionwavelength equal to or smaller than 1.4 μm.
 2. An optical fiberaccording to claim 1, wherein a cutoff wavelength is set to be equal toor smaller than 1.5 μm at a length of 2 m, and a bending loss is set tobe equal to or smaller than 5 dB/m at a diameter of 20 mm in thewavelength band of 1.5 μm.
 3. An optical fiber according to claim 1,wherein the optical fiber comprises: multiple glass layers adjacent toeach other and having different compositions; and at least three glasslayers formed inside a reference layer as a reference of a refractiveindex distribution among these multiple glass layers; wherein a maximumrefractive index of a first glass layer formed on an innermost side ofthe optical fiber is set to be higher than the refractive index of saidreference layer, a minimum refractive index of a second glass layer as asecond layer from an inner side of said optical fiber is set to be lowerthan the refractive index of said reference layer, and a maximumrefractive index of a third glass layer as a third layer from the innerside of said optical fiber is set to be higher than the refractive indexof said reference layer.
 4. An optical fiber according to claim 3,wherein a glass layer having a refractive index higher than that of thereference layer is arranged between the third glass layer and thereference layer.
 5. An optical fiber according to claim 3, wherein aglass layer having a refractive index lower than that of the referencelayer is arranged between the third glass layer and the reference layer.6. An optical fiber according to claim 3, wherein a relative refractiveindex difference Δ1 of the maximum refractive index of the first glasslayer with respect to the reference layer is set to 0.5% or more and0.6% or less, a relative refractive index difference Δ2 of the minimumrefractive index of the second glass layer with respect to saidreference layer is set to −0.4% or more and −0.1% or less, and arelative refractive index difference Δ3 of the maximum refractive indexof the third glass layer with respect to said reference layer is set to0.1% or more and 0.4% or less.
 7. An optical communication systemcharacterized in that an optical fiber according to claim 1 is appliedas an optical transmitting path.